Coordinate measurement tables of the type mentioned above are used in high-precision coordinate measuring devices for measuring substrates in the semiconductor industry. A coordinate measuring device having such a coordinate measurement table is described in the article “Maskenmetrologie mit der Leica LMS IPRO für die Halbleiterproduktion” (Mask Metrology with Leica LMS IPRO for Semiconductor Production) by K.-D. Röth and K. Rinn, Mitteilungen für Wissenschaft und Technik, Vol. XI, No. 5, pp. 130-135, October 1997. This measuring device serves for high-precision measurements of the coordinates of the edges of a structural element on a substrate, for example a mask and a wafer.
German Patent No. DE 101 40 174 A1 describes an improved coordinate measurement table of the type mentioned above and a coordinate measuring device for high-precision measurement of the coordinates of the edges of a structural element on a substrate, such as a mask and a wafer. The coordinate measuring device comprises a coordinate measurement table of the type mentioned above, which is displaceable horizontally in the x-direction and in the y-direction. It serves for receiving a substrate having structures whose edge coordinates are to be measured. Furthermore, a separate interferometer optical measurement path is associated with each coordinate axis (x, y) of the measurement table. On two sides of the measurement table, which are perpendicular to each other, measurement mirrors are mounted, which are located at the ends of the two interferometer optical measurement paths. The position of the measurement table may be determined interferometrically by means of the two measurement mirrors. The coordinate measurement table comprises a drive unit having a friction rod and a motor for each coordinate axis. The motor contacts one side of the friction rod with its motor shaft, while a pressure roller contacts the other side of the friction rod. At least one spring is provided, which biases the pressure roller, the friction rod and the motor shaft against each other with a pressing force. This results in the motor shaft frictionally engaging the friction rod so that the rotational movement of the motor is converted into a linear movement of the friction rod.
A further coordinate measurement table is described in German Patent No. DE 102 36 239 A1. A drive axis with a friction rod and a motor is provided for each coordinate axis. The motor contacts one side of the friction rod with its motor shaft, and a pressure roller contacts the other side of the friction rod. Furthermore, a pressure spring serves for biasing the mentioned parts against each other, whereby the motor shaft engages the friction rod frictionally and converts the rotational movement of the motor into a linear movement of the friction rod.
U.S. Pat. No. 4,378,709 discloses a friction drive for positioning a table. The friction bar is also driven directly by the motor shaft or the friction wheel. The friction bar is supported by a support roller on one side. On the other side of the friction bar, the friction wheel is supported by two support wheels. The support rollers are biased against the friction bar by a means. Automatic adaptation of the frictional force is not possible.
German Published Application No. DE 2005 036 718 A1 discloses a friction drive. The friction drive includes a friction wheel and a counterpart which is at least intermittently in frictional engagement with the friction wheel. There is further provided a force element designed to press the friction wheel and the counterpart against each other. The force element includes an air conduit for conducting a directed air flow 60 and at least one opening in the air conduit. The conducted air flow generates an air pressure in the area of the opening, which causes the frictional engagement. This arrangement cannot guarantee either that the friction wheel will adapt to the required load changes of the table movable in the X-coordinate direction and in the Y-coordinate direction.
International Patent Application No. WO 2007/010364 A2 discloses a friction drive. However, the document does not relate in any way to a drive or a linear drive of a friction bar. It only describes two wheels co-operating with each other, wherein one drives the other by friction drive.
Japanese Patent Abstract No. 2002-310264 A discloses a method and a device for controlling a friction drive provided with a soft means. For this purpose, a wheel co-operating with the other one is provided with a movable bar pressing the wheel into closer contact with the second wheel. The document does not disclose any arrangement as it is required in the present invention.
Coordinate measuring devices of the type mentioned above serve for determining the coordinates with a reproducibility in the range of less than 5 nm. Since this measurement accuracy, as mentioned above, depends significantly on the x/y positioning accuracy and the height process accuracy of the measurement table, extremely high requirements have to be met by the construction of the measurement table. Since, for relatively large masses to be moved, the frictional connection between motor and friction rod may be partly lost, a significant part of the originally desired positioning accuracy may be lost due to the associated slip. Undesired slipping effects may also occur during very fast advancing movements and high table accelerations.